BoilerFaultCodesThe 6 most common faults
Your boiler has switched itself off because the water pressure inside the system is too low for it to run safely.
Locate the filling loop (usually a silver braided hose with small valves underneath the boiler).Slowly open the valves until you hear water flowing into the system.Watch the pressure gauge and close the valves tightly once the needle reaches between 1.0 and 1.2 bar.Reset the boiler using the control knob if the fault code persists.
Your boiler is failing to light the burner, meaning it cannot produce heat or hot water because the fuel isn't igniting.
Check if your gas supply is turned on and other gas appliances are workingCheck if you have credit on your gas meterPress the 'Reset' button on the boiler control panel
Your boiler has likely overheated, causing a safety switch to trip and shut the system down to prevent damage.
Check that all radiator valves are fully open to allow water flowCheck the system pressure gauge and top up to 1.5 bar if lowRestart the boiler using the reset button/dialBleed all radiators to remove trapped air
Your boiler is struggling to detect a flame, which prevents it from firing up to provide heating or hot water.
Check that your gas emergency control valve is in the 'on' positionVerify if other gas appliances in your home, such as a hob, are working correctly to rule out a wider gas supply issueTry resetting the boiler by turning the selector knob to the 'R' position for a few seconds
Your boiler has stopped working because water is not moving around the system properly, which is preventing the unit from heating up safely.
Check that your system pressure is between 1.0 and 1.5 bar on the pressure gauge.Ensure all radiator valves and isolation valves under the boiler are fully open.Gently bleed your radiators to remove any trapped air that might be blocking flow.
Your boiler has detected that waste water cannot drain away properly, usually due to a blocked or frozen pipe outside.
Check the plastic pipe leading from the boiler to the outside drain for signs of ice or debris.Thaw any frozen sections of the external pipe using a hot water bottle or a cloth soaked in warm water.Ensure the internal trap or siphon is clear of debris if it is accessible without opening the sealed boiler casing.
on the Sime Murelle Pro HE 25 HO
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Sime shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue